(1) Field of the Invention
The present invention relates to a control method and a control apparatus for a variable wavelength optical filter that selectively separates optical signals of desired wavelengths from wavelength division multiplexed (WDM) signal light including a plurality of optical signals of different wavelengths. In particular, the present invention relates to a control technique for a variable wavelength optical filter for when collectively processing selective separation of optical signals of multiple wavelengths.
(2) Description of the Related Art
Recently, with the explosive increase in data communication demand mainly on Internet traffic, higher capacity and ultra-longer distances for backbone networks are being demanded. Furthermore, since services used by users are becoming multifarious, there is a demarid to simultaneously realize an economical network having high reliability and flexibility.
Currently, by wavelength division multiplexing transmission (WDM) techniques and optical amplification techniques, higher capacity and ultra-longer distances have been drastically progressed, enabling a decrease in transmission path cost. However, in the case of increasing information processing ability of a network node following higher speed of transmission signal and larger capacity, in conventional photoelectric conversion and electrical switching methods, there is caused an increase in node cost and a larger scale. From the above background, for more economical and smaller size nodes, development of optical add drop multiplexing (OADM) devices and optical cross connect (OXC) devices is anticipated, which replace large scale electronic circuits with optical components, to perform various processing in optical path units of an optical wavelength region.
Of these devices, a large number of optical function devices are used, such as an optical switch having functions for switching light on or off, for attenuating light, for switching to 1×n, and the like, or a wavelength filter for dividing signal light for each wavelength.
Among the aforementioned optical function devices, a device that can collectively process optical signals of a desired plurality of wavelengths from WDM signal light, is an important key device for realizing an OADM. Such a device that can collectively process is, for example, a device that can collectively block or drop multi-wavelength optical signals. More specifically, as such a device, devices such as for example, an acousto-optic tunable filter (AOTF), a fiber Bragg grating (FBG) filter, and the like are well-known.
FIG. 14 is a diagram showing one example of a conventional network configuration of OADM nodes using AOTFs.
Each OADM node of the network configuration as shown in FIG. 14, is required to have a function for selectively dropping an optical signal of a desired wavelength or a function for selectively blocking an optical signal of a desired wavelength. Collective dropping of optical signals of a plurality of wavelengths is a function which is required at a node (node 1 in FIG. 14) of a position where two or more ring nets or networks are overlapped with each other, and is necessary when optical signals of a plurality of wavelengths are sent from one network to the other network. Collective blocking of optical signals of a plurality of wavelengths is a function which is necessary in the case of blocking the passage of, for example, optical signals of a wavelength which needs to be terminated, or optical signals of a wavelength which overlaps with that of an optical signal to be added, amongst optical signals passing through within a node. At each OADM node, it is important to be able to drop and add optical signals for arbitrary wavelengths, in order to flexibly operate the network. For this purpose, it is also necessary to perform the aforementioned collective processing for optical signals of arbitrary wavelengths, and a variable wavelength optical filter such as an AOTF having a wavelength variable function, is useful.
FIG. 15 is a diagram showing a configuration example of the OADM device used for each node in FIG. 14. The OADM device in FIG. 15 includes an OADM unit (W) and an OADM unit (P) respectively corresponding to a work channel and a protection channel of a network. At each OADM unit, drop processing, block processing and add processing are respectively performed for WDM signal light being transmitted. In the drop processing in this configuration example, a part of input WDM signal light is separated by an optical coupler on an input side, and further branched corresponding to the required number of wavelengths, and then an optical signal of a desired wavelength is selectively dropped from the respective branched lights, using a drop type AOTF. In the block processing, WDM signal light having passed through the optical coupler on the input side is sent to a reject type AOTF, so that the passing of an optical signal of a desired wavelength is blocked. In the add processing, an optical signal of a desired wavelength to be added to WDM signal light at this node is wavelength multiplexed, and then sent to an optical coupler on an output side to be added to WDM signal light having passed through the reject type AOTF.
Incidentally, in the case where desired optical signals of a plurality of wavelengths are selectively separated using a variable wavelength optical filter, it is necessary to completely make the transmission central wavelength of filtering characteristics coincide with the aforementioned desired plurality of wavelengths. If the transmission central wavelength of the variable wavelength optical filter does not coincide with the wavelengths of optical signals, deterioration of rejection level, erroneous blocking of optical signals of other channels or the like, occurs in the block processing. Moreover, an increase in insertion loss, erroneous dropping of optical signals of other channels, or the like, in the variable wavelength optical filter, occurs in the drop processing, and this is fatal to the results of processing executed at the OADM nodes.
Generally, the wavelength of a transmission light source using a semiconductor laser (laser diode, LD) or the like has fluctuations. Moreover, for a variable wavelength optical filter itself, fluctuations occur in the transmission central wavelength due to a change with time, environmental variations, control errors or the like. Therefore, in order to realize a stable operation of OADM node, a tracking control detecting errors caused by the aforementioned wavelength fluctuations to feeds back to the control of the variable wavelength optical filter is indispensable.
This tracking control, for example, in the case of block processing, takes out an optical signal which has a complementary relationship with an optical signal whose passage is obstructed, as monitor light from the variable wavelength optical filter, and controls the variable wavelength optical filter so that the monitor light becomes a maximum. Furthermore, in the case of drop processing, it branches a part of the dropped light dropped by the variable wavelength optical filter, as monitor light, and controls the variable wavelength optical filter so that the monitor light becomes a maximum.
Such a tracking control of a variable wavelength optical filter is often performed individually, for example, for each wavelength to be selectively separated. In this case, only an error to the corresponding wavelength is detected and the result is fed back to the control of the corresponding transmission central wavelength. When block processing or drop processing for optical signals of multiple wavelengths is collectively performed, since multiple wavelength components of the aforementioned monitor light are also output collectively, there occurs a necessity for specifying which wavelength in the monitor light the detected error signal corresponds to.
As a method for specifying the corresponding wavelength of an error signal, there are methods such as for example, a method of adding a pilot tone (or dithering) to a drive signal corresponding to the wavelength being a control object, or a method of adding to a drive signal, dithering of different frequencies for each of the respective wavelengths. By each of these methods, it becomes possible to discriminate corresponding wavelengths of error signals. However, if the number of wavelengths for collective processing is increased so that the number of monitor lights to be collectively output is increased, power of the monitor lights other than wavelengths intended to be tracking controlled is increased, and the components thereof become noise for error signals. Therefore, in the case where detection accuracy of error signals is reduced due to an increase in the number of wavelengths for collective processing, there is caused a possibility that the tracking control of a variable wavelength optical filter becomes difficult.
Moreover, besides the above, for example, the reason why the tracking control of the variable wavelength optical filter becomes difficult is that, if consideration is given to saturation of an amplifier of a light receiver and an electronic circuit used for light reception processing of monitor light to make the dynamic ranges of these wider in advance so as to avoid saturation, since an amplitude of the error signal detected becomes relatively smaller, an S/N ratio of the error signal is deteriorated.
More specifically, consideration is given to, for example, a tracking control in the case of collective blocking optical signals of 10 waves using a rejection type AOTF. In this case, by applying to the rejection type AOTF, RF signals of 10 waves having frequencies set respectively corresponding to the wavelengths of each optical signal to be blocked, it becomes possible to block the passage of optical signals of 10 waves. In the tracking control at this time, 9 wave RF signals of the 10 wave RF signals are made to be fixed while adding dithering etc. to only the one wave RF signal, and a feedback control is performed on this RF signal, and this control is sequentially executed for all of the 10 wave RF signals.
In such a tracking control, for example as shown in FIG. 16, a monitoring circuit used for light reception processing of monitor lights corresponding to optical signals of 10 waves for which passage is to be blocked by the rejection type AOTF, is necessary to be designed so that it is not saturated even when it receives monitor lights of 10 wave components. Therefore, compared to a monitoring circuit used for a tracking control in the case of blocking only optical signals of one wave, in the monitoring circuit corresponding to 10 waves, a light reception voltage to be allocated to one wave becomes necessarily small. That is, as the maximum wavelength number the passage of which can be blocked by the rejection type AOTF is increased, a monitoring circuit is designed in expectation of such maximum wavelength number. Therefore, the light reception voltage to be allocated to one wave is decreased, making it difficult to accurately determine power variations of the monitor light corresponding to the channel which executes the tracking control (the reject ch. 6 in the example of FIG. 16).
Furthermore, as another reason why the tracking control of the variable wavelength optical filter becomes difficult, for example, it is possible that fluctuations of other wavelengths, which are close to a frequency of dithering to be added to the drive signal, cause deterioration in detection accuracy of the error signal. Moreover, since monitor lights corresponding to multiple wavelengths are collectively received, there is also a possibility of occurrence of power variations due to wavelength variations and polarization variations of each monitor light. Thus, it is also considered that such power variations are combined with the error component due to tracking, making the tracking control difficult.
As one method to solve the aforementioned problems related to the tracking control of variable wavelength optical filters, there is considered for example a method of separating the monitor lights of each wavelength output together corresponding to collective processing of optical signals of a plurality of wavelengths, for each wavelength by using a separately prepared optical filter, to detect error signals corresponding to each separated monitor light. However, in order to realize this method, for example, a fixed filter such as an arrayed waveguide grating (AWG) or a variable wavelength optical filter are newly required. Moreover, it also becomes necessary to respectively provide a light receiver and an electronic circuit corresponding to multiple monitor lights separated for each wavelength. Therefore, there is the drawback in leading a larger size and a higher cost for a variable wavelength optical filter inclusive of a controller.